1. Field of the Invention
The invention relates to an improved system for measuring the velocity of fluid streams utilizing a pressure transducer.
2. Description of the Prior Art
About 100 million bales of cotton are produced annually in the world of which 18-20 million are produced in the United States. After the harvesting operation, the seedcotton is taken to a cotton gin where one of the key processing steps is performed, the separation of the lint from the seeds and other contaminants. The predominant method of conveyance of the product through the cotton gin processing system is by pneumatic conveyance. Optimal operation of many of processing systems in the cotton gin can be improved through the monitoring of the air velocity.
Air velocity sensing in process control systems is predominantly performed by hot wire anemometry, however in some applications where the fluid lines are subjected to high levels of particulate contamination, the hot wire anemometer's calibration can be affected due to the build up of the contaminants on the hot wire sensor. A common alternative to the hot wire anemometer is to utilize a pressure sensor to determine the air velocity by measuring the time of flight of a pressure wave and thereby determine the velocity of the pressure wave. The velocity of the pressure wave provides a measure of both the speed of sound, a function of air temperature and density, as well as the velocity of the gas. In practice, the measurement is typically measured in both the upstream and downstream directions which allows for the quantification of the air velocity as well as the sonic velocity and thereby the molecular density of the fluid. As this also allows for the removal of the effects of the density from the measurement, this leads directly to a measure of the fluid velocity. This type of measurement is commonly referred to as a sonic flow meter.
A primary advantage of the sonic flow meter is that it doesn't require an invasion of the measurement space. As such, installation of a sonic flow system does not require a shutdown of the system being monitored nor does it require cutting into the fluid lines. Additionally, since the sonic flow meter does not require physical contact between the measuring apparatus and the liquid whose rate is being measured, there is no possibility that the sonic apparatus will alter the flow being measured or will be adversely affected by the chemical nature of the fluid being monitored. A number of different approaches have been taught in the art for sensing and measuring the motion of fluids utilizing principles of sonic velocity measurement.
One of the simplest techniques for measuring the fluid velocity utilizes a Doppler shift technique. In practice, the Doppler technique transmits a continuous stable frequency sinusoidal wave to an acoustic transducer that produces an acoustic pressure wave that is directed toward oncoming or outgoing particles that reflect the pressure wave back to the transducer. As the particles are moving relative to the stationary transducer, a Doppler frequency shift is imparted to this pressure wave. Upon reception of the returned wave, the frequency of the transmitted wave is compared to the received wave with the frequency difference providing a direct measure of the relative velocity between transducer and the particles. The disadvantage to the Doppler technique is that it only works in fluids with a significant amount of particulate contamination. Use of this system is also subject to errors when measuring streams having a mix of particulates of varying sizes. In such streams, the particulates of different sizes will impart different velocities onto the returned pulse due to the particulates traveling at different velocities due to the varying drag force, which is proportional to the wetted area of the particle. Thus, the mean particle size has a direct influence on the obtained measurement which affects the application's accuracy, as it is directly related to the uniformity and predictability of the suspended particles in the fluid stream. For use in cotton gins, this is an undesirable characteristic due to its inherent variability and unpredictability.
One of the most commonly practiced techniques for measuring fluid velocity utilizing pressure transducers is the direct time of flight measurement of either a single pulse or a burst of a successive set of pulses, U.S. Pat. Nos. 5,060,506, 6,568,281, 4,515,021, 4,308,754, and 3,575,050 are primary examples disclosed in their entirety by reference herein. The primary disadvantage to this technique is that for the close distances that are typical in these transducers, the requirements to directly and accurately measure the time of flight of a pulse transmission, as varied by the fluid velocity, leads to the requirements of extraordinarily fast clocks in order to resolve the time of flight to a sufficient level of resolution. For instance, in the measurement of air velocity across a typical range utilized by cotton gins (9.144 m/s (30 ft/s) to 12.192 m/s (40 ft/s)); the delta time difference between the low speed versus the high speed leads to a time difference of 22.2 μs. Given we'd like some dynamic range with the measurement, in order to obtain a 10 bit accurate measurement the measurement needs to be resolved with a 5.4 ns clock. This situation becomes even more exacerbated when the fluid is a liquid as the sonic velocity is over 4 times this speed. Thus, the time base for a typical liquid application at 10 bit resolution would have to be increased to over 2.8 GHz. Thus, this technique doesn't lend itself to an accurate, low cost solution.
Other problems associated with the direct time of flight measurement lay in the difficulty in the detection of the arrival time of an ultrasonic pulse passing through a fluid as it is affected by turbulent flow conditions in high Reynolds number flows as well as turbulence cause by entrained particles as well as changes in fluid temperature, pressure, and composition which leads to a received pulse of varying amplitude as well as shape. Thus, an ultrasonic pressure wave propagating through a fluid can be subjected to a wide variety of different and rapidly varying amounts of attenuation. In order to overcome this difficulty, many patents have focused on the development of automatic gain amplifiers to compensate. While this works, it further complicates an already difficult measurement.
Another commonly practiced technique transmits a continuous wave (CW) single frequency signal to the receiving transducer. Upon reception at the receiving transducer, the received signal is compared to an internal reference that is used to drive the transmitting transducer. As the internal signal is transmitted electrically, there is very little delay compared to the signal that is transmitted through the fluid via the pressure transducers. By measuring the phase difference between the internal reference signal and the pressure transmitted signal, it is possible to accurately determine the fluid velocity. By utilizing a measurement of the phase difference between the transmitted signal and the receiving signal, a much more accurate measurement than that provided by the time of flight technique, is obtained. One of the original patents utilizing this technique was U.S. Pat. No. 3,861,211 by Dewan.
Unfortunately, one of the fundamental limitations of the phase determination method is due to the inability of a phase difference measurement to resolve phase differences greater than 360 degrees since a sine wave repeats itself every 360 degrees. This leads to a problem referred to as “phase ambiguity”. Thus, this technique imposes limitations upon the span of the velocity as the total time delay measurement becomes a function of the transmitted frequency. When this is coupled with the fact that currently there are few low cost transducers available and they are frequency dependant with only a few frequency choices currently available, these issues to render this technique unusable in many applications.
Another primary problem with the phase difference measurement technique lays in it's inability to remove the effects of standing waves in a closed cavity. Thus, in practice, this technique is best utilized in open air structures where standing waves can be minimized. This need to avoid standing waves lays in the need to avoid receiving more than one signal that has taken different paths from the transmitter to the receiver as this leads to alterations in the direct path signal by the multi path signals. As the multipath signals are the same frequency as the direct path signal, the reception of the multipath signals in conjunction with the direct path signal alters both the amplitude as well as the phase of the original signal. Thus, despite the advances in the measurement of fluid velocity, there remains a critical need for production of a low cost sensor which can remove the effect of the multipath signals from the received direct path signal, while maintaining the accuracy of the direct time of flight methodology.